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Enclosure Thermal Control 25 August 2003 ATST CoDR Dr. Nathan Dalrymple Air Force Research Laboratory Space Vehicles Directorate.

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Presentation on theme: "Enclosure Thermal Control 25 August 2003 ATST CoDR Dr. Nathan Dalrymple Air Force Research Laboratory Space Vehicles Directorate."— Presentation transcript:

1 Enclosure Thermal Control 25 August 2003 ATST CoDR Dr. Nathan Dalrymple Air Force Research Laboratory Space Vehicles Directorate

2 Enclosure Thermal Control Function: Suppress seeing If a surface is the same temperature as the surrounding air, that surface introduces no seeing Seeing is caused by temperature differences

3 Requirements 1.Suppress enclosure seeing a.Racine experiment:  = 0.15  T i - T e ) 1.2 b.Ford analysis:  =  T s - T e  1.2 c.IR HB aerodynamic analysis:  =  T  V,  d.Bottom line: requirements on surface-air  T, interior- exterior  T, and wind flushing 2.Provide passive interior flushing to equalize interior and exterior temperatures and to suppress structure and mirror seeing Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, 1991.

4 Error Budgets (nm) Exterior budgetInterior budget nm10 nm arcsec0.02 arcsec arcsec0.025 arcsec

5 IR Handbook Seeing Analysis Given layer thickness and  T, we can estimate . Wavefront variance Gladstone-Dale parameter Fluctuating densityLine-of-sight correlation length Layer thickness Phase variance Surface-air temperature difference Blur angle Strong/weak cutoff ~ 2 rad Ref: Gilbert, Keith G., Otten, L. John, Rose, William C., “Aerodynamic Effects” in The Infrared and Electro- Optical Systems Handbook, v. 2, Frederick G. Smith, Ed., SPIE Optical Engineering Press, 1993.

6 IR Handbook Seeing Analysis (cont.) Layer thickness (mks units): L: upstream heated length (m)  T: average temperature difference over upstream length (˚C) V: wind speed (m/s) Buoyancy termHydrodynamic term Assume: If  T < 0 then buoyancy term does not contribute to layer thickness.

7 Shell Seeing, Diffraction- Limited Error Budget Blue contours: rms wavefront error (nm) Acceptable operating range, assuming no AO correction. AO correction will extend the “green” area. = 500 nm

8 Shell Seeing, Seeing-Limited Error Budget Blue contours: 50% encircled energy (arcsec) Acceptable operating range = 1600 nm

9 Shell Seeing, Coronal Error Budget Blue contours: 50% encircled energy (arcsec) Acceptable operating range = 1000 nm

10 Dome Seeing (Inside/Outside Air  T) Correlation by Racine (1991) Approximate error budget Approximate  T requirement Need lots of passive flushing! Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, 1991.

11 IR Handbook aerodynamic treatment Correlation of Racine (1991) IR Handbook aerodynamic treatment Good seeing from KE test Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, BBSO Dome Seeing Experiments

12 Bad seeing from KE test BBSO Dome Seeing Experiments

13 A Nighttime Comparison: Gemini Dome 1 Duct exhaust fan on, low-moderate wind (3 - 5 m/s)  T = -3 ˚C Acceptable seeing observed with shell subcooled by 3 ˚C.

14 Bottom Line Requirements Enclosure skin temperature needs to be subcooled by up to 3 ˚C Interior air temperature needs to be within 0.5 ˚C of ambient outside air Need large passive flowrate to flush interior

15 Skin Energy Balance We want to use this term to control the skin temperature [~0 W/m 2 ] [377 W/m 2 ] [374 W/m 2 ] [98 W/m 2 ] [~100 W/m 2 ] Quantities vary by location on dome and weather conditions

16 Skin Thermal Control System Concept Concept Features: 1.White oxide paint a.Large  b.Small  s 2.Chilled skin a.Air b.Liquid (EGW) 3.Insulation prevents interior from being chilled by skin coolant

17 Shutter: air cooled, optional water cooling on lower end h air ~ 8 W/m 2 -K h H2O ~ 100 W/m 2 -K Enclosure support wall: water cooled if present h H2O ~ 100 W/m 2 -K Oblique skin panels: air cooled, h ~ 5 W/m 2 -K Sun-facing skin panels: air or water cooled h air ~ 5 W/m 2 -K h H2O ~ 100 W/m 2 -K Option: use fins on skin underside to increase effective area Skin Thermal Control System Concept (cont.)

18 Skin Cooling System Flow Loop Insert diagram here

19 MuSES Modeling: Validation at Gemini North Validation

20 Skin Thermal Control System Performance MuSES snapshot at 1430LT, 30 April 2003, Mauna Kea Wind speed = 0.5 m/s Ambient air T e = 7 – 8 ˚C Air Cooling Only on Skin ESW Water Cooled Most of surface is acceptable Sun-facing areas are ~ 5 ˚C hotter than ambient Surfaces that see cold sky subcool

21 MuSES snapshot at 1430LT, 30 April 2003, Mauna Kea Wind speed = 0.5 m/s Ambient air T e = 7 – 8 ˚C Air & Water Cooling Nearly all of surface is acceptably cool Sun-facing areas cooled with water Surfaces that see cold sky subcool Skin Thermal Control System Performance (cont.)

22 Cooling Requirements Next steps: Fan and system curves Heat exchanger specs Chiller specs Time response of fluid volume At peak heat load, surface cooling requires: Air-cooled skin: 56 kW Water-cooled skin: 18 kW Lower shutter: 14 kW Air-cooled shutter: 18 kW Total for carousel: 106 kW Enclosure support wall: 104 kW Grand total: 210 kW (60 tons)

23 Flushing System Concept 42 vent gates 168 m 2 flow area, each side

24 Flushing System Performance

25 Active Interior Ventilation Gemini volume flowrate: 10 enclosure volumes/hour (150,000 m 3 /hr) This flowrate on the smaller hybrid gives V ~ 0.2 m/s average Directed flow can give V~0.5 – 1 m/s over much of structure Fans may be mounted remotely or on carousel

26 Active Ventilation Issues Fan blades heat air  seeing Require homogenizing screens, cooling coils downstream of fans May not be simple to mount all this on carousel  possible to mount remotely

27 Shell Seeing Performance Blue contours: rms wavefront error (nm) Red: average  T of skin, front skin, shutter, lower shutter, ESW Most of the dome surface will give acceptable seeing Back of shutter subcools. May need to add water cooling there as well.


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